What is DESIGN FOR TESTABILITY

What is testability?

•Controllability: This is the ability to set circuit nodes to a certain states or logic values.

•Observability: The ability to observe the state or logic values of internal nodes of the circuits.

If Area increases so the Logic complexity also increases and yield decreases. In such a case for a fixed fault coverage, the defect level also increases. So,DFTguaranty to increase the fault coverage of the manufactured devices. Manufacturing processes of ICs are very defective. A very few percentage near about the 20 percent of the manufactured ICs are good. But the requirement for high quality ICs are more. So, if we design a workable system, it becomes very helpful for us for manufacturing high-quality ICs. But we must also be able to test the system to a degree which ensures that the ICs which are manufactured are fully and properly functional. Design for Testability is design technique that add certain features of testability to a microelectronic hardware product design. The benefits of the added features are that it is easy to develop new ICs and apply manufacturing tests on the designed ICs and hardware and helpful in finding either they are working correctly or not.

Applications of DFT

DFTcan be applied at several steps in the hardware manufacturing process. In some cases it is also used for hardware maintenance in the customer’s environment. The tests are done by test programs which are executed inAutomatic Test Equipment (ATE)and for the system maintenance, inside the assembled systems itself. These tests are also able to find the diagnostic information about the nature of the encountered test fails. The diagnostic information is helpful in locating the sources of the failure.DFTplays a vital role in the development of test programs and as an interface for test application and diagnostics. Automatic test pattern generation, orATPG, is much easier if appropriateDFTrules and suggestions have been implemented during the manufacturing process.ATPGis an electronic design automationtechnology which is used to find an input sequence, which is applied to a digital circuit, so that the testers are enable to distinguish between the correct circuit behaviour and the faulty circuit behaviour accrued because of defects. The generated pattern is used to test semiconductor devices which are manufactured. In some cases it also assist in determining the cause of failure.

There are different types ofDFTs are used known as Ad-Hoc and Structural. StructuralDFTis further classified i) Scan Methods and ii) Built in Self test.

Benefits ofDFT

DFThas these benefits:

Ø Fault coverage area is increases.

Ø Test generation (development) time is decrease.

Ø Test length time is decreases.

Ø Test Memory is also decrease.

Ø Test application time is decreases.

Ø Support a test hierarchy system in manufacturing chips, boards and subsystems etc.

Original Structure Modified structure

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